Under normal physiological conditions, TGF-Beta1 is found predominantly in latent forms, which show no biological activity unless they are first activated in vitro by denaturants or extremes of pH. We have identified and characterized a naturally occurring latent TGF-Beta1 complex. Cells in culture and degranulating human platelets secrete TGF-Beta1 in a latent complex that consists of mature TGF-Beta1, non-covalently associated with the remainder of the TGF-Beta1 precursor, "pro" sequence, and a third unidentified protein of 135 KDa. We have further shown that TGF-Beta1 made by recombinant expression systems using the entire coding sequence of the human TGF-Beta1 gene is also biologically latent, and that this complex is identical to the natural cellular complex, except that it lacks the 135 KDa component. This demonstrates that the TGF-Beta1 precursor "pro" region alone is sufficient to confer latency on TGF-Beta1. This region is now referred to as the TGF-beta1 latency-associated peptide (LAP). We have purified the TGF-Beta1 LAP from recombinant sources, complexed it with (125)I-TGF-Beta1 and analyzed the pharmacokinetics of the resultant latent complex in rats. We have shown that, providing the LAP is correctly sialylated by the expression system, the plasma half-life of TGF-Beta1 in the latent complex is 109 min., compared with just 2.7 min. for active, uncomplexed TGF-Beta. This suggests that whereas endogenous active TGF-Beta probably has very local autocrine or paracrine action, endogenous latent TGF-Beta may have a more endocrine mode of action. The more favorable pharmacokinetcs of the latent TGF-Beta1 also suggest that this may be the form of choice for human clinical use.